1. Field of the Invention
The present invention relates to a switching control circuit.
2. Description of the Related Art
Various types of electronic equipment use a DC-DC converter for generating an output voltage of a target level from an input voltage. FIG. 2 is a diagram illustrating a general configuration of a step-down DC-DC converter. A DC-DC converter 100 includes an N-channel MOSFET 110, Schottky barrier diodes 111, 112, an inductor 113, capacitors 114, 115, resistors 116, 117, a control circuit 118, a level shift circuit 119, and an inverter 120.
An input voltage VIN is applied to a drain of the N-channel MOSFET 110 through a terminal IN, and when the N-channel MOSFET 110 is turned on, an input voltage VIN is applied to the inductor 113, the capacitor 114 is charged, and an output voltage VOUT is raised. Thereafter, when the N-channel MOSFET 110 is turned off, by virtue of energy accumulated in the inductor 113, an electric current is passed through a loop including the Schottky barrier diode 111, the inductor 113, and the capacitor 114, the capacitor 114 is discharged, and the output voltage VOUT is lowered. In the DC-DC converter 100, the output voltage VOUT is controlled so as to reach the target level by turning on/off the N-channel MOSFET 110 by the control circuit 118 so that a feedback voltage VFB obtained by dividing the output voltage VOUT by the resistors 116, 117 reaches a predetermined level.
Moreover, the DC-DC converter 100 uses the N-channel MOSFET 110, in which ON resistance and loss are smaller than those in a case of a P-channel MOSFET, as a transistor for applying the input voltage VIN to the inductor 113. In a case where the N-channel MOSFET 110 is used as such, when the N-channel MOSFET 110 is turned on, a voltage of the source of the N-channel MOSFET 110 gets close to the input voltage VIN. Thus, in order to keep the N-channel MOSFET 110 on, a voltage higher than the input voltage VIN by a threshold voltage VTH of the N-channel MOSFET 110 is required to be applied to the gate of the N-channel MOSFET 110. Moreover, in order to keep a state in which the ON resistance of the N-channel MOSFET 110 is sufficiently small, a voltage higher than the input voltage VIN by the order of 5V, for example, is required to be applied to the gate of the N-channel MOSFET 110.
Then, a method of using a bootstrap voltage in order to turn on the N-channel MOSFET 110 is generally employed (See Japanese Patent Laid-Open Publication No. 2008-141832, for example). In the DC-DC converter 100, a voltage VREG applied to a terminal REG is applied to the capacitor 115 through the Schottky barrier diode 112 and a terminal BC, so that a bootstrap voltage VBT is generated. Here, assuming that the voltage VREG is 5V and each forward voltage of the Schottky barrier diodes 111, 112 is 0.3V, such a state is considered that the N-channel MOSFET 110 is off and an electric current is passed through the loop including the Schottky barrier diode 111, the inductor 113, and the capacitor 114. In this case, a voltage VSW of a terminal SW is −0.3V, a voltage VBC of a terminal BC is 4.7V, and the bootstrap voltage VBT across the capacitor 115 results in 5V. Therefore, if the N-channel MOSFET 110 is turned on and the voltage VSW becomes equal to VIN, VBC=VIN+VET. Then, the level shift circuit 119 performs level shift of a control signal outputted from the control circuit 118 based on the voltage VBC and the inverter 120 uses the bootstrap voltage VBT as a driving voltage, and thus, the N-channel MOSFET 100 can be kept on.
In the DC-DC converter 100 as above, the terminal IN and the terminal BC might be short-circuited due to adhesion of dust or the like. Here, assume such a case that the terminal IN and the terminal BC are short-circuited when the bootstrap voltage VBT is 5V. When the N-channel MOSFET 110 is turned on, an electric current is passed from the terminal IN to the terminal SW through the N-channel MOSFET 110. For example, assuming that the input voltage VIN is 15V, the current flowing through the N-channel MOSFET 110 is 1 A, and the ON resistance of the N-channel MOSFET 110 is 0.2Ω, the voltage VSW of the terminal SW is 14.8V. At this time, since the terminal IN and the terminal BC are short-circuited, a current path is formed from the capacitor 115 to the N-channel MOSFET 110, and thus, the capacitor 115 is discharged.
Then, when the capacitor 115 is discharged, the bootstrap voltage VBT is lowered, and thus, the N-channel MOSFET 110 is turned off. Even if the N-channel MOSFET 110 is turned off, the inductor 113 tries to continue passing the current, and the voltage VSW of the terminal SW becomes −0.3V. On the other hand, since the terminal IN and the terminal BC are short-circuited, the voltage VBC of the terminal BC is 15V. Therefore, a voltage between the terminal BC and the terminal SW is 15.3V. Thus, the voltage of 15.3V is also applied to the inverter 120, and assuming that the withstand voltage of the inverter 120 is 7V, for example, such a state occurs that the withstand voltage of the inverter 120 is exceeded. Moreover, if the voltage applied to the gate of the N-channel MOSFET 110 from the inverter 120 reaches the order of 15V, the withstand gate-source voltage of the N-channel MOSFET 110 might be exceeded.